A suspension apparatus for a vehicle includes a suspension spring, and a damper interposed between a sprung member and an unsprung member. The damper damps vibrations of the sprung member and the unsprung member generated, for example, when the vehicle travels over an uneven portion of a road surface. The suspension spring absorbs (buffers) the above-mentioned vibrations. That is, the suspension apparatus absorbs (buffers) externally input vibrations, and serves as a damping force generation apparatus which generates damping force so as to damp the vibrations.
There has been known a suspension apparatus which includes a damper capable of changing its damping force characteristic in accordance with the traveling state of a vehicle. The damping force characteristic of such a damper is variably controlled by an actuator which operates in accordance with a control input calculated by a feedback controller designed on the basis of a predetermined control theory.
When the damping force characteristic of the damper is variably controlled, a nonlinear H-infinity control theory is applied as a control theory for calculating the magnitude of a variable portion of the damping force such that riding quality is improved. For example, Japanese Patent Application Laid-Open (kokai) No. 2001-1736 discloses a damping force controller which utilizes a closed loop control system which includes a generalized plant designed through addition of a nonlinear weight to a mechanical motion model representing an equation of motion of a suspension apparatus, and a state feedback controller for the generalized plant. The state feedback controller is designed to solve a nonlinear H-infinity control problem; i.e., is designed such that the L2 gain of the closed loop control system becomes less than a positive constant γ. In the damping force controller, a variable damping coefficient is calculated on the basis of a control input obtained by the state feedback controller designed in the above-described manner.